I. Field of the Invention
This invention relates generally to a system for measuring the ventricular volume of a human or animal heart by using impedance plethysmography and more particularly to a method for matching the sensed length of the impedance catheter to the length dimension of the ventricle.
II. Discussion of the Prior Art
In the Salo U.S. Pat. No. 4,674,518, which is assigned to the assignee of the present invention, there is described a method and apparatus for making instantaneous measurements of the ventricular volume by utilizing an intracavity electrical impedance catheter. That catheter comprises a tubular sheath having a plurality of spaced surface electrodes (ring electrodes) disposed sufficiently near the distal end of the catheter so that those electrodes effectively span the length dimension of the chamber whose volume is to be measured. To obtain accurate measurements of ventricular volume, it is imperative that one be able to locate the particular pair of sensing electrodes located at the transition between the ventricular chamber and the atrium or aorta, depending upon which ventricular chamber is involved. When this has been accomplished, the volume between each of the electrode pairs may be computed from measured impedance signals and then these volumes may be summed to yield the total ventricular volume. Because there is large variability in the length of the left (and right) ventricle over the total patient population, some means must be provided for matching the total distance spanned by the surface ring electrodes on the catheter to the length of the ventricle being measured.
The aforereferenced Salo U.S. Pat. No. 4,674,518 references the published work of Jan Baan of the Netherlands. In the work leading to those publications, a number of catheters with electrodes of different spacing were manufactured and the appropriate catheter selected under fluoroscopy. Since the length of the ventricle cannot be accurately estimated without actually introducing a catheter into the chamber, this technique often required the introduction of more than one catheter. Either a measuring catheter with radio opaque markers is positioned in the chamber first and the desired catheter length determined under fluoroscopy or else one of the impedance catheters is introduced and later replaced if it proves to be of the wrong length.
Such a trial and error approach at determining appropriate catheter length is not only time consuming but increases the risk of damage to the blood vessels through which the catheter is routed in achieving placement in the ventricle.